Conventionally known is a double-link piston crank mechanism equipped with an upper link connected at one end through a piston pin to a piston, a lower link connected through an upper pin to the other end of the upper link and connected to a crankpin of a crankshaft, and a control link rockably supported at one end by an engine body side and connected at the other end through a control pin to the lower link.
A large combustion pressure received by the piston is inputted from an upper-pin bearing portion to the lower link through the piston pin, the upper link, and the upper pin. At the same time, to balance with the previously-noted combustion load, loads are produced in a crankpin bearing portion and a control-pin bearing portion, respectively. Therefore, the bearing pressures of these bearing portions are severer as compared to a general single-link reciprocating engine. Hence, to prevent abrasion or seizing from occurring, maintaining of an adequate lubricating state is required.
For instance, Patent document 1 discloses a double-link piston crank mechanism having an oil passage formed in a crankshaft, a crankpin oil passage formed in a crankpin, a lower-link oil passage formed in a lower link, and a pin-boss oil passage positioned on the extended line of the lower-link oil passage as viewed in an axial direction of the crankshaft and formed in an upper-link pin boss part onto which an upper pin is rotatably fitted. The double-link piston crank mechanism is further configured such that the crankpin oil passage and the lower-link oil passage are brought into fluid-communication with each other at a prescribed crank angle so as to feed part of lubricating oil jetted from the lower-link oil passage through the pin-boss oil passage to an upper-pin bearing portion.
However, assuming that position-setting of the lower-link oil passage with respect to the pin-boss oil passage is improper, the anti-seizing property of the upper-pin bearing portion tends to lower owing to an insufficient supply of lubricating oil. For instance, when the operating range of the internal combustion engine is a high speed range, it is difficult to jet out lubricating oil in the direction in which the lower-link oil passage opens owing to a force of inertia. In particular, in a configuration such that a pin-boss oil passage is positioned on the extended line of a lower-link oil passage, adequate lubricating oil cannot be necessarily fed to the pin-boss oil passage.